1. Field of the Invention
The present invention generally relates to a flash device capable of achieving a high speed synchronism by intermittently firing for a long time.
2. Description of the Prior Art
In general, the intensity of light emitted from a flash tube used in the flash device depicts a generally peak-shape, rapidly increasing immediately after the actual firing and terminating in a very short time, for example, a few milliseconds. Because of this, in a photographic camera having a focal plane shutter mechanism, at a higher shutter speed than the flash synchro speed (for example, 1/60 second), the synchronized flash firing cannot be achieved. In other words, in the camera having the focal shutter mechanism, the shutter does not open full at the higher shutter speed than the flash synchro speed and a slit defined between the first and second blinds traverse a frame of photographic film, therefore, whenever the flash device is fired, only a portion of the frame of photographic film is exposed by the flash light. The consequence is that a photographic picture the subject of which has been uniformly exposed under flash lighting cannot be obtained.
In order to obviate the above discussed problem, an attempt has been made to repeatedly firing the flash tube in a pulsated fashion to emit light of a character similar to that exhibited by a FP-class flash bulb so that, even at the shutter speed higher than the flash synchro shutter speed, the synchronized flash lighting can be attained. An example of this flash device is disclosed in the Japanese Laid-open Patent Publication No. 61-98334 published in 1986.
It has, however, been found that, in the prior art flash device capable of achieving a high speed synchronization, since the intensity of light emitted (effective value) is fixed, a flash synchronized photo-taking in which a shutter speed and an aperture are selected at the will of a photographer cannot be performed, the reason for which will now be discussed.
The illumination intensity Efl of an object to be photographed which is illuminated by the flash device can be expressed as follows. EQU Efl=I/D.sup.2 ( 1)
wherein I represents the intensity (effective value) of light emitted from the flash device and D represents the distance from the flash device to the object. Accordingly, the illumination intensity E of the object can be expressed as follows if the illumination intensity under ambient lighting is expressed by Es. EQU E=Efl+Es (2)
Therefore, assuming that the object is a completely diffusible plane having a reflectance .rho., the brightness B of the object can be expressed by the following equation. EQU B=.rho./.pi..multidot.E (3)
When the equation (3) is expressed in terms of APEX system, it results in. EQU Bv=log.sub.2 (B/NK) (4)
wherein each of N and K represents a constant.
Since according to the APEX system, the aperture value Av, the shutter speed Tv, the film speed Sv and the brightness Bv of the object have the following relationship. EQU Av+Tv=Bv+Sv (5)
the following equation establishes. EQU Av+Tv=Sv+log.sub.2 (.rho./NK.pi.).multidot.(Es+I/D.sup.2) (6)
In general, since the film speed is fixed and since the reflectance .rho. and the illumination intensity Es under ambient lighting are also fixed once the object to be photographed is determined, the sum of the shutter speed Tv and the aperture value Av is a function of the intensity I of flash light and the distance D.
Assuming that the intensity I of flash light is fixed and once the distance D is determined, the sum of the shutter speed Tv and the aperture value Av are fixed. Accordingly, if the shutter speed Tv (or the aperture value Av) is set to a desired value, the aperture value Av (or the shutter speed Tv) is naturally determined.
As hereinabove discussed, with the prior art flash device, the flash photography cannot be performed with the shutter speed and the aperture value selected as desired.